[dana934]

A friend without a computer is wondering
Whether there's a noise cancellation system
That can stand by itself in the middle of an
Apartment near a freeway. She really can't
Picture herself wearing headphones all the time
She's at home. Anyone know of a free-standing
Device that recognizes and cancels sound?

[dmatos]

Unfortunately, that type of device is just not possible. Sound travels in air as a compression wave, but if you want, you can think of it like a wave in water. There are peaks and troughs. You can also stack waves on top of each other, like little rippleson big waves, for example. Earphone/headphone based noise cancellation devices work by generating the exact opposite sound wave (a peak where there is a trough, and vice versa), which, when added to the incoming sound wave, exactly cancels it out.

Unfortunately, there are only two places where this can work. Either at the source of a sound, or at the receiver of the sound. Think about dropping a pebble in a pond. It makes a series of rings. If you drop another pebble in a different spot, in some locations on the pond, the ripples will be smaller. In others, they will be larger. There is no way to cancel the sound waves over a large area.

The best that she can do is try to block the windows so the freeway sound does not enter as loudly, buy a white noise generator and try to drown out the cars, or just plain old get used to it. I lived in an apartment near a freeway once, and it took about two months before I reached the state where the traffic noise was no longer noticeable.

[bhayne]

I believe you are thinking of some device that can match the frequency and produce wave 180 degrees so that destructive interference cancels the sound. Yes.

Road noise is quite complex (each car's road noice will produce its own frequency) and I know there are electronic filters that remove unwanted sound (as from video tape) within a frequency range.

For real live noise dmatos is probably right (or at least it is top secret CIA stuff). However, there is a much simpler solution- noise absorbing material.

This is material that has special designed surface and is made of special material to absorb the unwanted noise. I have lived by an airport and many homes not only have this in their attics but have a special film on the windows to absorb the noise.

[dana934]

Great! Where do you get it? I've told my friend I'll help her
Stick egg cartons on the walls.

[bhayne]

I'd go to any home improvement store. Home Depot, etc.

[dana934]

Thank you for the response. I now have very good news: My friend has found a much better apartment to rent and will be moving in a week and a half--without earplugs. Thanks again.